Restricted service telephone system



Oct. 16,1956 D. N. MacDONALD RESTRICTED SERVICE TELEPHONE SYSTEM Filed July 20, 1951 INVENTOR. Duncan N. MacDonald Attorney RESTRICTED Snnvtcn TntaPnoNE SYSTEM Duncan N. MacDonaid, Evanston, 111., assignor to Automatic Electric Laboratories, inn, Chicago, EL, a corporation of Delaware Application .iuiy 20, 1951, Serial No. 237,710

26 Claims. ca. 179-48) This invention relates to automatic telephone circuits and, more particularly, to restricted service on a two-wire line. It permits only certain subscribers to reach a special line which may be a loudspeaker system for making announcements in a plant or store, or it may be an executives private incoming line, or it may be an outgoing line where it is desired to limit the number of persons who may make outside calls.

An object of this invention is to provide a special line for the use of a limited number of subscribers.

Another object of this invention is to provide a decoding circuit which will react to a mutilated chain of pulses, closing a circuit to a special line when the pulses are received in the proper sequence but which will prevent switch-through if the pulses are received in an improper sequence.

Another object of this invention is to provide certain subscribers with coding means for reaching a restricted line through a decoding circuit.

A further object of this invention is to make unnecessary the fine timing of numerous slow-to-operate relays.

A still further object of this invention is to make it impossible for an unauthorized party to reach the special line by dialing a plurality of smaller digits in an attempt to duplicate the coded pulse chain transmitted by the means provided authorized subscribers.

One feature of this invention is a decoding circuit comprising a chain of relays each of which has an associated contact, which contacts are in series one with another and must all be closed before a circuit may be completed connecting a subscriber to a special line. An alternative to this chain of relays is a Z-level rotary switch with the special line connected to one contact on each bank of the switch and the calling line connected to the wipers of the switch. The motor magnet is operated by pulses transmitted by a coding means located at a calling substation.

Another feature of this invention is a means for permitting the coding of the impulses transmitted by an impulse sender when a certain digit is dialed to provide restricted service. Certain subscribers are provided with a special calling device which enables them to reach a restricted line unavailable to other subscribers. This calling device may be a special dial.

A further feature of this invention is a slow-to-release relay which, on deenergization, connects the wiper of a rotary switch to ground and thereby either energizes a chain relay or steps a second rotary switch if the wiper of the first switch pauses on the proper contact. This feature makes it unnecessary to use slow to operate chain relays. Simple relays may be used instead.

A still further feature of this invention is a means for preventing unauthorized subscribers from reaching the special line. If, by dialing a plurality of small digits, an attempt is made by an unauthorized subscriber to imitate the mutilated chain of pulses transmitted by a specially equipped substation the connecting circuit will not switch through because a vital relay deenergizes between digits 'ice and that relay cannot be reenergized until the rotary switch has returned to its normal position.

Fig. 1 shows a substation connected through a central ofi'ice exchange to the special line equipment of the present invention.

Figs. 2 and 3 show separate variations of the special line equipment of Fig. 1.

Fig. l is a schematic diagram showing at the left a standard substation such as that described in Pye Patent 2,214,259 equipped with a special dial for privileged subscribers. The dial is similar to that described in Ostline Patent 2,366,647, except that the cams such as 45 in Fig. 1 of said patent are arranged to close shunt springs 16 in Fig. l of this invention to shunt impulse springs 4 at the instant they are opened so as to prevent the breaking of the line circuit for certain of the impulses of a chain, the shut springs creating this shunt only when a locking pull-button 15 has been operated by the privileged subscriber. At the right of Fig. l is shown a special line 99 which can be reached only by privileged subscribers who have special dials, a plurality of chain relays which connect the subscriber to the special line, a decoding circuit which operates the connecting means in response to a signal from a special dial, and a circuit which receives the impulses and operates the decoding means.

This special line can be reached only upon switch through by the code circuit which responds only to a coded chain or sub-series of pulses transmitted by the substation with the special dial. The code circuit is reached as any other circuit would be reached by dialing its call number. The circuit as shown is connected to a contact ofa connector but it could be attached directly to a level of a selector. if the latter is used, an entire level of the selector must be busied while the circuit is in use. Any phone can reach this circuit, but only those with special dials can be switched through to the special line.

When the line is seized by the connector, current flows from ground through relay 19 of the code circuit, through connector 29, selector 39, line finder 49, and line circuit 59 and through the loop circuit of the substation by way of the following path: transmitter S, induction coil 6, impulsing springs 4 and contact 2 of hookswitch 1. From there the current flows back through the line to relay l0 and negative battery.

The substation is shown with the hookswitch in talking position. When dial 109 is moved to an ofi-normal position shunt springs 7 and 8 are closed. When impulsing springs 4 are closed during dialing, current flows through the substation through the following path: from the positive line conductor through shunt springs 8, impulse springs 4, and contact 2 to the negative line conductor.

When the proper number has been dialed and the code circuit is seized, pull-button 15 is pulled up by the subscriber to a position where it looks until the handset is replaced to thereby render the cam springs 16 eiiective to shunt impulse springs 4. The calling party then dials zero or some other predetermined number and a cam in the dial causes cam springs 16 to close for predetermined intervals, blanking out certain pulses, thereby coding the impulse chain. When cam springs 16 are closed and impulsing springs 4 are open current flows through the following path: from the positive line conductor through shunt springs 8, pull-button 15, cam springs 16, contact 2 to the negative line conductor. Thus a mutilated chain or sub-series of pulses will be transmitted to the code circuit.

When the code circuit is seized, relay 10 is operated, opening contact 12 and closing contact 11. Current flows from ground through contact 11, operating relay 20' which opens contact 22 and closes contacts 21 and 23,

V busying the line by grounding the .C conductor through contact 23. When the first pulse is received, relay is deenergized, opening contact 11 and closing contact 12. Relay is slow-to-release and does not deenergize until the calling party replaces his handset. Current flows from ground through contacts 12, .21 and 42, and through relay and magnet in parallel to negative battery. Relay 30 operates, closing contacts 31 and opening contacts 32 and 33. Current now flows from ground through contacts 12, 21, 31, through relay .30 and motor magnet 40 of rotary switch 200 to negative battery.

At the end of the pulse, relay 10 is energized, again opening contact 12 and closing contact 11. Motor magnet 40 is deenergized and steps wiper 215 of switch 290 to contact 201 as well as moving off-normal springs 41 and 42 to an oil-normal position, opening contact 42 and closing contact 41. 'Relay 30 is slow-to-release and does not release during normal pulsing but will release when the dial reaches its normal position. Once contact 42 has been opened in this manner, contact 31 offers the only path for current to relays 30 and motor magnet 40. If relay 310 releases, opening contact 31, there is no way to The special line remains seized untilthe calling party hangs up. When he replaces his handset, the loop circuit is opened and relay 10 is deenergized, closing contact 12 and opening contact 11. Relay 20 deenergizes after an interval, opening contact 21 and closing contact 22. Springs 41 and 42 are still in an oft-normal position so current flows from ground through contacts 12, 22, 43, and 41 to motor magnet 40 and negative battery. Magnet 40 energizes and interrupts its own circuit by opening contact 43. Magnet 40 thereupon deenergizes, 'stepping switch 200 and closing contact 43. With contact 43 closed, the circuit to magnet 40 is again closed and the magnet is energized, deenergized and'reenergized until Wiper 215 of switch 200 reaches its normal position at which time the oif-normal springs return to normal and contact 41. is opened, preventing further stepping of the switch. All relays are now deenergized, the switch has horned, all circuits are open, and the code circuit is again ready to be seized.'

send any more pulses to this relay and the motor magnet.

The subscriber must hang up and start again. This prevents unauthorized subscribers from simulating a mutilated V 8. Pulses other than, or in addition to, pulses 2, 5 and 8 may be blanked out by using a different cam'in connection with cam spring 16 in dial 100. As will be seen, if pulses other than 2, 5 and 8 are chosen, different contacts on rotary switch 200 will be connected to the chain relaysshown in Fig. l and Fig. 2 or to rotary switch 300 shown in Fig. 3. Relay80 is slow-to-operate and does not operate unless wiper 215 pauses on contact 201 of switch 200 while a pulse is blanked out. operated by current which flows from ground through wiper 215, contact 201, and relay S0 to negative battery. Relay 80 operates, closing contact 81 which locks up relay 80, current flowing from ground through. contacts 23and 81 through relay 80 to negative battery.

' At the beginning of the third pulse, relayll) deenergizes, contact 12 closes and current flows from ground through contacts 12, 21 and 31 to magnet 40 and negative battery. At the end of the pulse relay 10 is energized, contact 12 is opened and magnet 40 is deenergized, stepping wiper 215 to contact 202. The fourth pulse produces the same re- Impulse 5 is blanked out by cam springs 16 as was Relay 80 is 7 impulse 2, causing wiper 215 to pause on contact 203 long lock itself up. As well as looking themselves up by closing contacts 81, 7.1, and 61, these relays preparea talking circuit from the substation to special line 90 by closing contacts 82, 72, and 62, and contacts 83,73, and 63;

. The talking circuit is completed when contacts 32 and 33 are closed which is accomplished in the following manner: When dial 100 reaches its normal position after pulsing, relay 10 is energized, opening contact 12. Relay.

30'releases, closing break contacts 32 and 33. If, as shown in Fig. 1, special line 90 isconnected to a speaker 9 9, no

. talking battery is necessary on the special line. "However,

if the line is connected to a telephone, talking battery must be applied to the special line. This is done in a well known manner by means of a line circuit 59 as shown connected to the calling line. a i

cuit in parallel with relay 230.

Fig. 2 shows another embodiment of this invention which, it is believed, is an improvement over Fig. .1. The

calling substation and line circuit and the switches for seizing the special line are not shown in Fig. 2 because they are the same as in Fig. 1.

The improvement shown in Fig. 2 is a different means the danger of missing one link in the chain which would result should one of the relays not operate soon enough. Instead, one slow-to-release relay 250 is added to the ciroperation of break contact 251 which connects wiper 415 to ground when closed. Relay 250 is timed so that it will not release during normal pulsing but will release it a pulse is omitted. As in Fig. 1 when the first pulse is received relay 230 is energized. Relay 230 closes contacts 231 and 234, energizing motor magnet 240, current tery. Relay 250, on energization opens contact 415 which is in series with wiper 215. Therefore, although wiper 415 is stepped and comes in contact'with contacts 401,

403, and 405 of switch 400 as it will do when any sub-.

scriber dials zero after calling this circuit, the chain relays will not be operated unless the wiper pauses on each of these contacts long enough for magnet 250 to release and close contact'251 and thereby connect the wiper to ground.

If the wrong pulses are omitted, the Wiper will pause on the Wrong contacts and the chain relays will not be energized. In Fig. 1, the wiper is grounded directly but in Fig. 2 it is grounded only if contacts 223 and 251 are closed. The homingcircui-t for switch 400 is the same in Fig. 2 as for switch 200 in Fig. l.

Fig. 3 showsa third embodiment of this invention. It is the same as Fig. 1 in all respects except that in place of the single level rotary switch 200 shown in Fig- 1 it has a 3-level rotary switch 200A and in place of the three chain relays 60, and 80, it has two-level rotary switch Wiper 515 operates exactly as does wiper 415 in Fig. 2 by means of a motor magnet such as 240, and wipers 325 V in Fig. 3, contacts 501, 503,

and 335 move therewith. and 505 are strapped and connected to motor magnet 310 of switch 300. Thus, when wiper 515 pauses oncontact 501 long enough for relay 250 to release, current flows from ground through contact 223, contact 251 which close when relay 250 releases, Wiper 515, contact 501 and motor magnet 310 to negative battery. j Then, when relay 250 is energized on the next pulse, contact 251 is opened and magnet 310 deenergizes and steps wipers 375 and 385 to contacts 371 and 3381. Likewise, w'heu wiper 515 pauses on contacts 503 and 505 of switch 200A,

This relay controls the wipers 375 and 385 of switch 360 are stepped to contacts 372 and 382, and then to 373 and 383. Contacts 373 and 383 are connected to special line 390. Wipers 375 and 385 are connected to contacts 327 and 337 of switch 203A. Wipers 325 and 335 must rest finally on contacts 327 and 337 in order to complete a talking circuit from the substation shown in Fig. 1 through condenser 292, contacts 233, Wiper 335, contact 337, wiper 385, contacts 383, line 399 and back through contact 373, wiper 375, contact 327, wiper 325, contact 232, and condenser 291 to the substation. Wipers 325 and 335 will rest on the proper contacts if zero is dialed and 3 pulses are blanked out. It is also necessary for switch-through that wipers 375 and 385 rest finally on contacts 373 and 383. This will happen only if motor magnet 310 receives 3 impulses from switch 200A and deenergizes after the third pulse.

Switch 360 homes in the same manner as switch 200. Ofi-normal springs 312 and interrupter springs 311 correspond to oli-normal springs 41 and interrupter springs 43 of switch 200. When the subscriber hangs up, contact 22 closes. Off-normal springs 312 and interrupter springs 311 are closed and current flows from ground through contacts 12, 22, 312, and 311 to motor magnet 31d and negative battery. Magnet 310 opens contact 311and interrupts itself. On deenergization, it steps switch 3%. This is repeated until it reaches home and the oflf-normal springs open, ending the stepping operation.

Other embodiments of this invention not shown include circuits with only two chain relays used in connection with special dials with only two lobes which blank out only 2 pulses, or circuits wherein wipers 375 and 385 are connected to contacts 326 and 336 instead of 327 and 337. If only 2 pulses are blanked out in a code circuit like that in Fig. 3, wipers 325 and 335 will rest on contacts 328 and 338 unless a number lower than zero is dialed. If wipers 375 and 385 are connected to contact 326 and 336, the number nine would be dialed if three pulses were blanked out and the number 8 would be dialed if only 2 pulses were blanked out.

While there has been described what is at present considered to be the preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim:

1. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means at said substation for mutilating said coded chain by blanking out only certain individual pulses, a special line, means for extending a connection from said substation to said special line, and means individual to said special line controlled by said mutilated chain of pulses for thereafter completing the connection of said substation with said special line, said connecting means comprising means for receiving said pulses, means for decoding said chain of pulses, and means for completing said connection only upon receipt of the properly mutilated chain of pulses.

2. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means at said substation for mutilating said coded chain by blanking out only certain individual pulses of the chain, a restricted line, means individual to said line responsive only to the receipt of a predetermined mutilated chain of impulses for completing a connection between said substation and said restricted line.

3. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving and decoding said pulses responsive to the receipt of a predetermined code signal for effecting a connection between said substation and said restricted line, said connecting 6 means comprising a plurality of series connected contacts and a plurality of chain relays controlling said contacts, said relays controlled by said receiving and decoding means.

4. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means at said substation for mutilating said coded chain by blanking out certain pulses, a restricted line, means individual to said line for receiving said pulses, a rotary switch individual to said line for decoding said pulse chain, and means controlled by said rotary switch for connecting said substation to said restricted line in response to a predetermined code signal.

5. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means at said substation for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch controlled by said receiving means and a plurality of chain relays controlled by said rotary switch, and means controlled by said decoding means for connecting said substation to said restricted line.

6. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch controlled by said receiving means and a plurality of chain relays controlled by said rotary switch, and means controlled by said decoding means for connecting said substation to said restricted line, said connecting means comprising a plurality of series-connected contacts controlled by said chain relays.

7. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch controlled by said receiving means and a plurality of slow-to-operate chain relays controlled by said rotary switch, and means controlled by said decoding means for connecting said substation to said restricted line, said connecting means comprising a plurality of series-connected contacts controlled by said chain relays.

8. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, and means responsive to the receipt of a predetermined code signal for effecting a connection between said substation and said restricted line, said connecting means comprising a plurality of rotary switches connected one to another, the first connected to said substation and the last to said restricted line.

9. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, and means responsive to the receipt of a predetermined code signal for efiecting a connection between said substation and said restricted line, said connecting means including a first rotary switch controlled by said receiving means having 3 levels of contacts and a second rotary switch controlled by the first having 2 levels of contacts, the first level of the first switch controlling the motor magnet of the second switch, the wiper on the first level of the second switch being connected to a contact on the second level of the first switch, the wiper on the second level of the second switch connected to a contact on the third level of the first switch, the wipers on the second and third levels of the first switch connected to the talking circuit of the substation, and a contacton each of the levels of the second switch connected to the '7 restricted line, the receipt of the proper code signal causing said rotary switches to connect said substation to said restricted line.

10. A telephone circuit comprising a substation having means for transmitting a code chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch, a plurality of relays'and a ground contact, said ground contact controlled by said receiving means and causing the wiper of said rotary switch to be connected to ground for predetermined intervals, the length and occurrence of which are determined by the code signal received, said relays connected each to a contact 'on the bank of said rotary switch, said relays operated by said wiper and ground contact, and means controlled by said chain relays for connecting said substation to said restricted line in response to a predetermined code signal.

11. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch, a plurality of relays and a ground contact, said ground contact controlled 'by said receiving means and causing the wiper of said rotary switch to be connected to ground for predetermined intervals the length and occurrence of which are determined by the 'code signal received, said relays connected each to a 9 contact on the bank of said rotary switch, said relays operated by said wiper andground contact, and means controlled by said chain relays for connecting said substation to said restricted line in response to a predetermined code signal, said connecting means comprising a plurality of series-connected contacts controlled by said chain relays.

12. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses,

means for mutilating said coded chain by blanking out certain pulses, a restricted line, means for receiving said pulses, means for decoding said chain of'p'ulses and for connecting said substation with said restricted line comprising a first rotary switch having 3 levels of contacts, and a second rotary switch having 2 levels of contacts, and means for connecting one of the wipers of the second switch to contacts on the first switch, one level or" the first switch controlling the motor magnet of the second switch and itself controlled by said receiving means.

13. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, which coding means includes a dial having a plurality of cams, a set of shunt springs, means for connecting said shunt springs at will to said transmitting means, said shunt springs operated by one of said cams to thereby form a shunt around the impulse springs thereby blanking out certainpulses, a restricted line, means for receiving said pulses, and means responsive to the receipt of a'predetermined code si nal for efiecting a connection between said substation and said restricted line.

14. A telephone circuit comprising a substation having means for-transmitting a coded chain of electrical pulses, which coding means includes a dial having cam springs which form a shunt around the impulse springs thereby blanking out certain pulses, a restricted line, means for receiving and decoding said pulsesresponsive to the receipt of a predetermined code signal for ellecting a connection between said substation antlsaid restricted line, said connecting means comprising a plurality of series-connected contacts and a plurality of chain relays controlling said contacts, said relays controlled by said receiving and decoding means. 7

15. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, which coding means includes a dial having cam springs which forma shunt around the impulse springs thereby blanking out certain pulses, and a restricted line, means for receiving said pulses, a rotary switch 'for do coding said pulse chain and means controhed bysaid rotary switch for connecting said substation to said re stricted line in response to a predetermined code signal.

16. A telephone circuit comprising asubstation having means for transmitting a coded chain of electrical pulses, which coding means includes a dial having cam springs which form a shunt around the impulse springs thereby blanking out certain pulses, and a restricted line, means 'for receiving said pulses, means for decoding said pulse chain comprising a rotary switch controlled by said receiving means and a plurality of chain relays controlled by said rotary switch, and means controlled by said decoding means for connecting said substation to said restricted line.

17. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses, which coding means includes a dial having cam springs which form a shunt around the impulse springs thereby blanking out certain pulses, and a restricted line, means for receiving said pulses, means for decoding said pulse chain com-prising a rotary switch controlled. by said re' ceiving means and a plurality of chain relays controlled by said rotary switch and means controlled by said decoding means for connecting said substation to said restricted line, said connecting means comprising a plurality of series-connected contacts controlled by said chain relays.

18. A telephone circuit comprising a substation having meansfor transmitting a coded chain of electrical pulses,

which coding means includes a dial having cam springs which form a shunt around the impulse springs thereby blanking out certain pulses, and a restricted line, means for receiving said pulses, means for decoding said pulse chain comprising a rotary switch controlled by said receiving means and a plurality of slow-to-operate chain relays controlled by said rotary switch and means controlled by said decoding means for connecting said substation to said restricted line, said connecting means comprising a plurality of series-connectedcontacts controlled by said chain relays.

19. A telephone circuit comprising a substation having means for transmitting a coded chain of electrical pulses,

which coding means includes afdial having cam springs which form a shunt around the impulse springs thereby blanking out certain pulses, a restricted line, means for receiving said pulses and means responsive to the receipt of a predetermined code signal ,foreffecting a connection between said substation and said restricted line, said connecting means including a first rotary switch controlled by i to the talking circuit of the substation and a contact on each of the levels of the second switch connected to the restricted line, therec'eipt of the proper code signal causing said rotary switches to connect said substation to said restricted line. Y

20. in a telephone system, a substation having a. line circuit and a dial associated therewith, impulse springs operated by a return of the dial to normal after each operation to open the substation line circuit, shunt means associated with the dial for preventing the opening of the line circuit by saidimpulsesprings only at predetermined points in its return so thatonly a'coded portion of the impulses is transmitted, connecting means at the substa- Lion, said connecting means controlledat will'by the sub scriber at said substation for rendering said shunt means eiiective.

21. A telephone circuit comprising a substation having means thereat for transmitting a coded chain of lectrical pulses, means integral with said pulse transmitting means for blanking out only certain individual pulses of said coded chain of pulses to thereby form a coded chain of pulses, means operable at will for rendering said last named means efiective, a restricted line, and means controlled by said coded chain of pulses for connecting said substation to said restricted line, said connecting means comprising means for receiving said pulses, means for decoding said chain of pulses and means for completing said connection only upon receipt of the properly coded chain.

22. A restricted line selector for connecting with a restricted line connected over an intermediate link to a substation having a transmitting means capable of transmitting a series of impulses, means integral to and controlled by said transmitting means for preventing some of the impulses within said series from being transmitted thereby producing a coded sub-series of impulses when said transmitting means is operated, said selector comprising means for receiving said impulses, means for de coding said impulse sub-series comprising a rotary switch controlled by said receiving means and actuating means controlled by said rotary switch, and means controlled by said decoding means for connecting said substation to said restricted line, said connecting means comprising a plurality of contacts controlled by said actuating means, said receiving means including a detecting means, said detecting means operated in response to receipt of said coded pulses for discriminating between the time intervals between pulses, means controlled by the operation of said detecting means if the time interval is correct to extend the pulses to the decoding means thereby preventing any unauthorized attempt to access said restricted line by particular manipulation of the dial to simulate a coded chain of pulses with unequal time spacings.

23. In a telephone system, a plurality of lines, substations on said lines, a plurality of switching means associated with each substation, means for transmitting a plurality of series of impulses, means in said switches operated by said plurality of series of impulses to extend a connection to the terminals of a called line, means at the substation for transmitting a further series of impulses having certain of the pulses blanked out, means individual to certain of said called lines for completing a connection to the substation on said certain lines, said last means operated responsive only to the receipt of said further series of impulses to complete said connection only when particular pulses have been blanked out.

24. A telephone circuit comprising a substation having means for transmitting a series of equally spaced impulses, means integral to said transmitting means, for preventing transmission of certain of said impulses within said series of impulses by blanking out only certain impulses therein, thereby transmitting a coded sub-series of impulses, a special line, and means on the line operated in response to receipt of said coded sub-series of impulses for connecting said substation to said special line, said connecting means comprising means for receiving said impulses, means for decoding said coded sub-series of impulses, and means for completing said connection upon receipt of the proper coded sub-series of impulses.

25. A telephone system including a plurality of substation lines and automatic switches for extending connections from a calling to a called one of said lines, an impulse sender on each line operated in accordance with a called number to transmit a plurality of series of impulses, means in said switches operated responsive to said series of impulses to extend a connection to a called one of said lines, means individual to certain of said calling lines, operable at will, to cause a further series of impulses to be divided into a coded sub-series of impulses and means individual to certain of said called lines, operable by said further series only when said further series of impulses is so divided, to complete a connection to said certain lines.

26. In a telephone system, subscribers lines, means for extending connections therefrom, an impulse sender for each line, said sender operable to transmit a plurality of series of pulses for controlling said connection extending means, a special line, certain of said substations having means thereat manually controlled at the will of the subscriber for causing one of said series of pulses to be transmitted as a coded sub-series of pulses by blanking out certain predetermined pulses within said one series of pulses, and means individual to said special line responsive only to said coded sub-series of pulses to complete a connection to said special line.

References Cited in the file of this patent UNITED STATES PATENTS 1,738,624 Wicks Dec. 10, 1929 1,954,967 Waller Apr. 17, 1934 2,187,945 Lomax Jan. 23, 1940 2,206,147 Zimmerman July 2, 1940 2,333,489 Peterson Nov. 2, 1943 2,339,376 Christian Jan. 18, 1944 2,518,210 Wicks Aug. 8, 1950 2,519,867 Wicks Aug. 22, 1950 

